Cut metal gate process for reducing transistor spacing

ABSTRACT

A semiconductor structure includes a substrate; an isolation structure over the substrate; a first fin extending from the substrate and through the isolation structure; a first source/drain structure over the first fin; a contact etch stop layer over the isolation structure and contacting a first side face of the first source/drain structure; and a first dielectric structure contacting a second side face of the first source/drain structure. The first side face and the second side face are on opposite sides of the first fin in a cross-sectional view cut along a widthwise direction of the first fin. The first dielectric structure extends higher than the first source/drain structure.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.17/588,883, filed Jan. 31, 2022, issuing as U.S. Pat. No. 11,721,544,which is a continuation of U.S. patent application Ser. No. 16/854,627filed Apr. 21, 2020, now U.S. Pat. No. 11,239,072, which is acontinuation of U.S. patent application Ser. No. 16/421,532 filed May24, 2019, now U.S. Pat. No. 10,651,030, which is a continuation of U.S.patent application Ser. No. 15/827,709 filed Nov. 30, 2017, now U.S.Pat. No. 10,319,581, the disclosures of which are herein incorporated byreference in their entireties.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs.

For example, when designing and manufacturing SRAM (static random accessmemory) cells having pull-up (PU) devices, pull-down (PD) devices, andpass-gate (PG) devices, it is common to form PU devices (e.g., PMOS) inone device region (e.g., in an n-well), and form PD and PG devices inanother device region (e.g., in a p-well). However, at least for the PUdevices, there is a concern that the spacing among them needs to besufficiently large so that epitaxial source/drain (S/D) features of thePU devices do not merge to cause short defects. On the one hand, havinglarge epitaxial S/D features are generally desirable for reducing S/Dcontact resistance. On the other hand, having large epitaxial S/Dfeatures also increases the spacing requirements among the PU devices,thereby undesirably reducing device integration. An object of thepresent disclosure seeks to resolve this issue, among others.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1A shows a top view of a semiconductor structure implemented with acut metal gate process, according to aspects of the present disclosure.

FIGS. 1B, 1C, and 1D show cross-sectional views of the structure in FIG.1A, in accordance with some embodiments.

FIGS. 2A, 2B, and 2C show a flow chart of a method for forming thestructure shown in FIGS. 1A-1D, according to aspects of the presentdisclosure.

FIGS. 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B, 8A, 8B, 9A, 9B, 10, 11,12A, 12B, 13, 14A, 14B, 15, 16, 17, and 18 illustrate cross-sectionalviews of a semiconductor structure during a fabrication processaccording to the method of FIGS. 2A-2C, in accordance with someembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present disclosure is generally related to semiconductor devices andfabrication methods, and more particularly to fabricating FinFETsemiconductor devices using a cut metal gate process that beneficiallyreduces the spacing requirements between adjacent fins such as fins forforming p-type FinFETs. A cut metal gate (CMG) process refers to afabrication process where after a metal gate (e.g., a high-k metal gateor HK MG) replaces a dummy gate structure (e.g., a polysilicon gate),the metal gate is cut (e.g., by an etching process) to separate themetal gate into two or more portions. Each portion functions as a metalgate for an individual transistor. An isolation material is subsequentlyfilled into trenches between adjacent portions of the metal gate. Thesetrenches are referred to as cut metal gate trenches, or CMG trenches, inthe present disclosure. A CMG process according to the presentdisclosure includes two exposure steps and two etching steps (so-called2P2E). The first exposure step and the first etching step are designedfor etching dielectric layers and those merged epitaxial S/D featuresthat need to be separated, without etching the metal gates. The secondexposure step and the second etching step are designed for etching themetal gates. By utilizing this 2P2E process, semiconductor fins can bearranged closer and epitaxial S/D features can be grown larger thantraditional devices. This simultaneously serves two purposes: increasingdevice integration by reducing spacing between semiconductor fins, andgrowing large epitaxial S/D features for reducing S/D contactresistance.

FIG. 1A illustrates a top view of a semiconductor device (orsemiconductor structure) 100. FIG. 1B illustrates a cross-sectional viewof the device 100 along the B-B line of FIG. 1A. FIG. 1C illustrates across-sectional view of the device 100 along the C-C line of FIG. 1A.FIG. 1D illustrates a cross-sectional view of the device 100 along theD-D line of FIG. 1A.

Referring to FIGS. 1A-1B, the device 100 includes a substrate 102, aplurality of fins 104 protruding out of the substrate 102 including fins104 a in a first device region 103 a and fins 104 b in a second deviceregion 103 b, an isolation structure 106 over the substrate 102 andbetween the fins 104, and a plurality of gate structures 112 disposedover the fins 104 and the isolation structure 106.

The fins 104 are oriented lengthwise along X direction and spaced fromeach other along Y direction perpendicular to the X direction. In thepresent embodiment, the fins 104 a are designed for forming p-typeFinFETs; and the fins 104 b are designed for forming n-type FinFETs. Thefins 104 a have an edge-to-edge spacing P1 along the Y direction. In anembodiment, P1 ranges from 20 to 30 nm, which is smaller thantraditional fin configurations where adjacent epitaxial S/D features areformed separately (not merged). In a particular embodiment, P1 isdesigned to be few nanometers greater than a resolution of a lithographyexposure tool, such as an extreme ultraviolet (EUV) exposure tool whoseresolution is about 13.3 nm in an embodiment. The smaller spacing P1advantageously increases device integration. Some of the fins 104 b areplaced close to each other for forming multi-fin transistors forboosting device performance. In the embodiment shown in FIG. 1A, thereare two groups of dual fins 104 b. The spacing between the fin 104 a anda nearby fin 104 b is P2, which ranges from 40 to 50 nm in anembodiment. The spacing between two groups of fins 104 b is P3, whichranges from 40 to 50 nm in an embodiment. In various embodiments, agroup of fins 104 b may include two (as shown), three, or more fins forforming multi-fin transistors.

The gate structures 112 are oriented lengthwise along the Y direction,and are spaced from each other along the X direction. The gatestructures 12 engage the fins 104 a and 104 b in their respectivechannel regions to thereby form FinFETs. In the present embodiment, thegate structures 112 engage the fins 104 a to form p-type FinFETs, whichmay be used for pull-up (PU) devices in SRAM cells; and the gatestructures 112 engage the fins 104 b to form n-type FinFETs, which maybe used for pull-down (PD) devices or pass-gate (PG) devices in SRAMcells. Due to the reduced spacing P1, the SRAM cells configured with thepresent PU, PD, and PG devices have a smaller area than traditional SRAMcells.

Still referring to FIGS. 1A-1B, the device 100 further includes S/Dfeatures 162, including S/D features 162 a and 162 b disposed over thefins 104 a and 104 b respectively. It is noted that not all of the S/Dfeatures 162 are illustrated in FIG. 1A for the sake of simplicity.Generally, S/D features 162 are disposed on each of the fins 104 intheir respective S/D regions. In an embodiment, the S/D features 162 ainclude p-type doped silicon germanium, while the S/D features 162 binclude n-type doped silicon.

The device 100 further includes a dielectric layer 114, includingdielectric features 114 a, 114 b, and 114 c. Particularly, thedielectric features 114 a are disposed between two rows of fins 104 a inthe device region 103 a, and the dielectric features 114 b and 114 c aredisposed between two groups of fins 104 b in the device region 103 b, aswell as between the device regions 103 a and 103 b. The dielectric layer114 fills in CMG trenches, and is therefore referred to as CMGdielectric layer 114. The CMG dielectric layer 114 is arrangedlengthwise along the X direction and separates some of the gatestructures 112 into at least two portions. In the present embodiment,the areas indicated by the dashed boxes 113 a and 113 b are processed byone exposure and etching process, while the areas indicated by thedashed boxes 113 c are process by another exposure and etching process.This aspect will be discussed in detail later. The dielectric features114 a are disposed within the dashed box 113 a and expand from one edgeof a gate structure 112 to an adjacent edge of the gate structure 112along the X direction. The dielectric features 114 b are disposed withinthe dashed boxes 113 b and expand from one edge of a gate structure 112to an adjacent edge of the gate structure 112 along the X direction. Thedielectric features 114 c are disposed within the dashed boxes 113 c andexpand from one edge of a gate structure 112 to an adjacent edge of thegate structure 112 along the Y direction. In the present embodiment, thedielectric features 114 b are wider than the dielectric features 114 calong the Y direction. The dielectric features 114 a, 114 b, and 114 cinclude the same dielectric material(s) in the present embodiment. Thewidth W1 of the dielectric features 114 a along the Y direction issmaller than P1 and ranges from 16 to 18 nm in an embodiment. In anembodiment, the width W1 is designed to be the same or slightly greaterthan the resolution of the lithography exposure tool, such as an EUVexposure tool whose resolution is about 13.3 nm.

Referring to FIG. 1B, the CMG dielectric feature 114 a is disposedbetween and in physical contact with two S/D features 162 a. In anembodiment, the interfaces 115 between the CMG dielectric feature 114 aand the two S/D features 162 a are two generally straight lines in thiscross-sectional view, whose straightness depends on the etching anddeposition processes that form the CMG dielectric features 114 a as willbe discussed later. In an embodiment, each of the interfaces 115 formsan angle ranging from 0 to 5 degrees with normal (Z direction) of a topsurface of the substrate 102. In an embodiment, the horizontal (alongthe Y direction) distances between the two interfaces 115 at differentheights (along the Z direction) are about the same or linearly andmonotonically decrease from top to bottom. In another embodiment, thetwo interfaces 115 are tilted toward each other from top to bottomregardless whether or not they are generally straight lines. Theinterfaces 115 are different from other facets of the S/D features 162a. The other facets are formed by epitaxial growth process and generallyfollow the crystalline orientation of the semiconductor material(s) ofthe S/D features 162 a, while the interfaces 115 are formed by etchingthe S/D features 162 a regardless of the underlying crystallineorientation.

Referring to FIG. 1C, each gate structure 112 includes a high-kdielectric layer 108 and a conductive layer 110 over the high-kdielectric layer 108. The conductive layer 110 includes one or morelayers of metallic materials. Therefore, each gate structure 112 is alsoreferred to as a high-k metal gate (or HK MG) 112. The gate structures112 may further include an interfacial layer (not shown) under thehigh-k dielectric layer 108. The CMG dielectric feature 114 c separatesthe gate structure 112 into left and right portions. The left portionengages a fin 104 a to form a transistor, and the right portion engagestwo fins 104 b to form another transistor.

Referring to FIG. 1D, in this cross-sectional view, the CMG dielectricfeature 114 a is in physical contact with only one S/D feature 162 a.The above discussion of the S/D feature 162 a, the interface 115, andthe CMG dielectric feature 114 a also applies to FIG. 1D.

The device 100 further includes one or more dielectric layers, such as acontact etch stop layer (CESL) 164 disposed over the S/D features 162and the isolation structure 106, and an inter-layer dielectric (ILD)layer 166 disposed over the isolation structure 106, the fins 104, thegate structures 112, and the CESL 164. The components of the device 100are further described below.

The substrate 102 is a silicon substrate in the present embodiment.Alternatively, the substrate 102 may comprise another elementarysemiconductor, such as germanium; a compound semiconductor includingsilicon carbide, gallium arsenide, gallium phosphide, indium phosphide,indium arsenide, and indium antimonide; an alloy semiconductor includingsilicon germanium, gallium arsenide phosphide, aluminum indiumphosphide, aluminum gallium arsenide, gallium indium arsenide, galliumindium phosphide, and gallium indium arsenide phosphide; or combinationsthereof.

The fins 104 may comprise one or more semiconductor materials such assilicon, germanium, silicon carbide, gallium arsenide, galliumphosphide, indium phosphide, indium arsenide, indium antimonide, silicongermanium, gallium arsenide phosphide, aluminum indium phosphide,aluminum gallium arsenide, gallium indium arsenide, gallium indiumphosphide, and gallium indium arsenide phosphide. In an embodiment, thefins 104 may include alternately stacked layers of two differentsemiconductor materials, such as layers of silicon and silicon germaniumalternately stacked. The fins 104 may additionally include dopants forimproving the performance of the device 100. For example, the fins 104may include n-type dopant(s) such as phosphorus or arsenic, or p-typedopant(s) such as boron or indium.

The isolation structure 106 may comprise silicon oxide, silicon nitride,silicon oxynitride, fluoride-doped silicate glass (FSG), a low-kdielectric material, and/or other suitable insulating material. Theisolation structure 106 may be shallow trench isolation (STI) features.Other isolation structure such as field oxide, LOCal Oxidation ofSilicon (LOCOS), and/or other suitable structures are possible. Theisolation structure 106 may include a multi-layer structure, forexample, having one or more thermal oxide liner layers adjacent to thefins 104.

The high-k dielectric layer 108 may include one or more high-kdielectric materials (or one or more layers of high-k dielectricmaterials), such as hafnium silicon oxide (HfSiO), hafnium oxide (HfO₂),alumina (Al₂O₃), zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃),titanium oxide (TiO₂), yttrium oxide (Y₂O₃), strontium titanate(SrTiO₃), or a combination thereof.

The conductive layer 110 includes one or more metal layers, such as workfunction metal layer(s), conductive barrier layer(s), and metal filllayer(s). The work function metal layer may be a p-type or an n-typework function layer depending on the type (PFET or NFET) of the device.The p-type work function layer comprises a metal selected from but notrestricted to the group of titanium nitride (TiN), tantalum nitride(TaN), ruthenium (Ru), molybdenum (Mo), tungsten (W), platinum (Pt), orcombinations thereof. The n-type work function layer comprises a metalselected from but not restricted to the group of titanium (Ti), aluminum(Al), tantalum carbide (TaC), tantalum carbide nitride (TaCN), tantalumsilicon nitride (TaSiN), titanium silicon nitride (TiSiN), orcombinations thereof. The metal fill layer may include aluminum (Al),tungsten (W), cobalt (Co), and/or other suitable materials.

The CMG dielectric layer 114 may include one or more dielectricmaterials, such as silicon nitride, silicon oxide, silicon oxynitride,fluoride-doped silicate glass (FSG), a low-k dielectric material, and/orother suitable insulating material; and may be formed by CVD (chemicalvapor deposition), PVD (physical vapor deposition), ALD (atomic layerdeposition), or other suitable methods.

The CESL 164 may comprise silicon nitride, silicon oxynitride, siliconnitride with oxygen (O) or carbon (C) elements, and/or other materials;and may be formed by CVD, PVD, ALD, or other suitable methods. The ILDlayer 166 may comprise tetraethylorthosilicate (TEOS) oxide, un-dopedsilicate glass, or doped silicon oxide such as borophosphosilicate glass(BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), borondoped silicon glass (BSG), and/or other suitable dielectric materials.The ILD layer 166 may be formed by PECVD (plasma enhanced CVD), FCVD(flowable CVD), or other suitable methods.

FIGS. 2A, 2B, and 2C illustrate a flow chart of a method 200 for formingthe semiconductor device 100 in accordance with an embodiment. Themethod 200 is merely an example, and is not intended to limit thepresent disclosure beyond what is explicitly recited in the claims.Additional operations can be provided before, during, and after themethod 200, and some operations described can be replaced, eliminated,or moved around for additional embodiments of the method. The method 200is described below in conjunction with FIGS. 3A-18 , which illustratevarious cross-sectional views of the semiconductor device 100 duringfabrication steps according to the method 200.

At operation 202, the method 200 (FIG. 2A) provides, or is providedwith, a device structure 100 having a substrate 102, fins 104 (includingfins 104 a and 104 b) protruding out of the substrate 102, and anisolation structure 106 over the substrate 102 and between the fins 104,such as shown in FIGS. 3A and 3B. Particularly, FIGS. 3A and 3B showcross-sectional views of the device structure 100 along the B-B line andthe C-C line of FIG. 1A, respectively. The various materials for thesubstrate 102, the fins 104, and the isolation structure 106 have beendiscussed above with reference to FIGS. 1A-1D.

In an embodiment, the substrate 102 may be a wafer, such as a siliconwafer. The fins 104 can be formed by epitaxially growing one or moresemiconductor layers over the entire area of the substrate 102 and thenpatterned to form the individual fins 104. The fins 104 may be patternedby any suitable method. For example, the fins 104 may be patterned usingone or more photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers, or mandrels, may then be used to pattern the fins104 by etching the initial epitaxial semiconductor layers. The etchingprocess can include dry etching, wet etching, reactive ion etching(RIE), and/or other suitable processes. For example, a dry etchingprocess may implement an oxygen-containing gas, a fluorine-containinggas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-containinggas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas(e.g., HBr and/or CHBR₃), an iodine-containing gas, other suitable gasesand/or plasmas, and/or combinations thereof. For example, a wet etchingprocess may comprise etching in diluted hydrofluoric acid (DHF);potassium hydroxide (KOH) solution; ammonia; a solution containinghydrofluoric acid (HF), nitric acid (HNO₃), and/or acetic acid(CH₃COOH); or other suitable wet etchant.

The isolation structure 106 may be formed by one or more deposition andetching methods. The deposition methods may include thermal oxidation,chemical oxidation, and chemical vapor deposition (CVD) such as flowableCVD (FCVD). The etching methods may include dry etching, wet etching,and chemical mechanical planarization (CMP).

At operation 204, the method 200 (FIG. 2A) forms gate structures 112engaging the fins 104. In an embodiment, the operation 204 includesdepositing the various layers of the gate structures 112 including thegate dielectric layer 108 and the conductive layer 110, and patterningthe various layers to form the gate structures 112 as illustrated inFIGS. 1A and 1C. In a particular embodiment, the operation 204 uses areplacement gate process where it first forms temporary (or dummy) gatestructures and then replaces the temporary gate structures with the gatestructures 112. An embodiment of the replacement gate process isillustrated in FIG. 2B including operations 204 a, 204 b, and 204 c,which are further discussed below.

At operation 204 a, the method 200 (FIG. 2B) forms temporary gatestructures 149 engaging the fins 104 such as shown in FIGS. 4A and 4B,which show cross-sectional views of the device 100 cut along the A-Aline and the C-C line of FIG. 1A, respectively. Referring to FIGS. 4Aand 4B, each temporary gate structure 149 includes an interfacial layer150, an electrode layer 152, and two hard mask layers 154 and 156. Theoperation 204 a further forms gate spacers 160 on sidewalls of thetemporary gate structures 149.

The interfacial layer 150 may include a dielectric material such assilicon oxide layer (e.g., SiO₂) or silicon oxynitride (e.g., SiON), andmay be formed by chemical oxidation, thermal oxidation, atomic layerdeposition (ALD), CVD, and/or other suitable methods. The gate electrode152 may include poly-crystalline silicon (poly-Si) and may be formed bysuitable deposition processes such as low-pressure chemical vapordeposition (LPCVD) and plasma-enhanced CVD (PECVD). Each of the hardmask layers 154 and 156 may include one or more layers of dielectricmaterial such as silicon oxide and/or silicon nitride, and may be formedby CVD or other suitable methods. The various layers 150, 152, 154, and156 may be patterned by photolithography and etching processes. The gatespacers 160 may comprise a dielectric material, such as silicon oxide,silicon nitride, silicon oxynitride, silicon carbide, other dielectricmaterial, or combinations thereof, and may comprise one or multiplelayers of material. The gate spacers 160 may be formed by depositing aspacer material as a blanket over the isolation structure 106, the fins104, and the temporary gate structures 149. Then the spacer material isetched by an anisotropic etching process to expose the isolationstructure 106, the hard mask layer 156, and a top surface of the fins104. Portions of the spacer material on the sidewalls of the temporarygate structures 149 become the gate spacers 160. Adjacent gate spacers160 provide trenches 158 that expose the fins 104 in the S/D regions ofthe device 100.

At operation 206, the method 200 (FIGS. 2A and 2B) forms source/drain(or S/D) features 162, such as shown in FIGS. 5A and 5B, which arecross-sectional views of the device 100 along the A-A line and the B-Bline of FIG. 1A, respectively. For example, the operation 206 may etchrecesses into the fins 104 exposed in the trenches 158, and epitaxiallygrow semiconductor materials in the recesses. The semiconductormaterials may be raised above the top surface of the fins 104, asillustrated in FIGS. 5A and 5B. The operations 206 may form the S/Dfeatures 162 separately for NFET and PFET devices. For example, theoperations 206 may form the S/D features 162 b with n-type doped siliconfor NFET devices and form the S/D features 162 a with p-type dopedsilicon germanium for PFET devices. In the present embodiment, some ofthe S/D features 162 merge together, such as shown in FIG. 5B.Particularly, two S/D features 162 a that are designed for twoindividual PFETs merge, and two S/D features 162 b that are designed fora multi-fin NFET also merge. Typically, two S/D features that aredesigned for two individual transistors (as opposed to a multi-fintransistor) are not allowed to merge together. To avoid the merging, thespacing between the two fins 104 a is typically designed to be greaterthan the lateral size of the S/D feature 162 a. This typically requireseither greater spacing (more area) for the two individual transistors orsmaller epitaxial S/D features. Neither is ideal because the formerwould reduce device integration and the latter would increase S/Dcontact resistance. The present disclosure improves over the typicalapproaches by initially growing S/D features 162 a large enough to mergeand then etches the merged S/D feature to separate them, which will bediscussed in details later.

At operation 208, the method 200 (FIGS. 2A and 2B) forms variousfeatures including a contact etch stop layer (CESL) 164 over the S/Dfeatures 162, and an interlayer dielectric (ILD) layer 166 over the CESL164, such as shown in FIGS. 6A and 6B, which are cross-sectional viewsof the device 100 along the A-A line and the B-B line of FIG. 1A,respectively. The CESL 164 may comprise silicon nitride, siliconoxynitride, silicon nitride with oxygen (O) or carbon (C) elements,and/or other materials; and may be formed by CVD, PVD (physical vapordeposition), ALD, or other suitable methods. The ILD layer 166 maycomprise tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass,or doped silicon oxide such as borophosphosilicate glass (BPSG), fusedsilica glass (FSG), phosphosilicate glass (PSG), boron doped siliconglass (BSG), and/or other suitable dielectric materials. The ILD layer166 may be formed by PECVD, FCVD, or other suitable methods. Theoperation 208 may perform one or more CMP processes to planarize the topsurface of the device 100, remove the hard mask layers 154 and 156, andexpose the electrode layer 152.

At operation 204 b, the method 200 (FIG. 2B) removes the temporary gatestructures 149 to form gate trenches 169, such as shown in FIGS. 7A and7B, which are cross-sectional views of the device 100 along the A-A andC-C lines of FIG. 1A, respectively. The gate trenches 169 exposesurfaces of the fins 104 and sidewall surfaces of the gate spacers 160.The operation 204 b may include one or more etching processes that areselective to the material in the electrode layer 152 and the interfaciallayer 150. The etching processes may include dry etching, wet etching,reactive ion etching, or other suitable etching methods.

At operation 204 c, the method 200 (FIG. 2B) deposits gate structures(e.g., high-k metal gates) 112 in the gate trenches 169, such as shownin FIGS. 8A and 8B which are cross-sectional views of the device 100along the A-A and C-C lines of FIG. 1A, respectively. The gatestructures 112 include the high-k dielectric layer 108 and theconductive layer 110. The gate structures 112 may further include aninterfacial layer (e.g., SiO₂) (not shown) between the high-k dielectriclayer 108 and the fins 104. The interfacial layer may be formed usingchemical oxidation, thermal oxidation, ALD, CVD, and/or other suitablemethods. The materials of the high-k dielectric layer 108 and theconductive layer 110 have been discussed above with reference to FIGS.1A-1D. The high-k dielectric layer 108 may include one or more layers ofhigh-k dielectric material, and may be deposited using CVD, ALD, and/orother suitable methods. The conductive layer 110 may include one or morework function metal layers and a metal fill layer, and may be depositedusing methods such as CVD, PVD, plating, and/or other suitableprocesses.

At operation 210, the method 200 (FIGS. 2A and 2B) forms one or morepatterned hard mask layers over the device 100, such as shown in FIGS.9A and 9B which are cross-sectional views of the device 100 along theB-B line and the C-C line of FIG. 1A, respectively. One hard mask layer170 is illustrated in this example. The hard mask layer 170 may includetitanium nitride, silicon nitride, amorphous silicon, yttrium silicate(YSiO_(x)), or other suitable hard mask material(s). In an embodiment,the operation 210 deposits the hard mask layer 170 using CVD, PVD, ALD,or other suitable methods, and subsequently patterns the hard mask layer170 to form openings 171. The openings 171 correspond to the dashedboxes 113 a and 113 b of FIG. 1A. The openings 171 expose the conductivelayer 110 and the ILD layer 166. In an example, the operation 210 mayform a patterned photoresist over the hard mask layer 170 by photoresistcoating, exposing, post-exposure baking, and developing. The patternedphotoresist provides openings corresponding to the boxes 114 a and 114 bof FIG. 1A. In a particular embodiment, the operation 210 uses a singleexposure process (e.g., using EUV exposure) to expose the photoresistlayer to have a latent image that includes the dashed boxes 113 a and113 b, and then develops the photoresist layer to provide the openings.Then, the operation 210 etches the hard mask layer 170 using thepatterned photoresist as an etch mask to form the opening 171. Theetching process may include wet etching, dry etching, reactive ionetching, or other suitable etching methods. The patterned photoresist isremoved thereafter, for example, by resist stripping.

At operation 212, the method 200 (FIG. 2A) etches the device 100 throughthe openings 171. The patterned hard mask layer 170 protects the rest ofthe device 100 from the etching process. In the present embodiment, theoperation 212 uses an etching process that is tuned to selectively etchthe ILD layer 166 and the S/D features 162 a without (orinsignificantly) etching the gate structures (e.g., HK MG) 112. Forexample, the operation 212 may perform a dry etching process usinghydrogen fluoride (HF) and ammonia, and may use argon gas as a carriergas. These etchants are selective to oxide (in the ILD layer 166) andsilicon or silicon-germanium (in the S/D features 162), and do not etchwell the conductive layer 110 in the gate structures 112. Referring toFIG. 10 which is a cross-sectional view of the device 100 along the B-Bline of FIG. 1A, the operation 212 extends the openings 171 down andthrough the ILD layer 166 and the S/D features 162 a and may extend theopenings 171 into the isolation structure 106. In the cross-sectionalview of the device 100 along the C-C line of FIG. 1A, the device 100remains about the same as shown in FIG. 9B because the etching processis tuned to not etch the conductive layer 110.

At operation 214, the method 200 (FIG. 2A) fills the trenches 171 withone or more dielectric materials to form the dielectric layer 114including the dielectric features 114 a and 114 b, and performs achemical mechanical polishing (CMP) process to remove the patterned hardmask 170 and to planarize the top surface of the device 100. Theresultant device 100 is illustrated in FIG. 11 . Since the sidewalls ofthe gate structures 112 contain metallic materials, at least the outerportion of the dielectric layer 114 (that is in direct contact with thesidewalls of the gate structures 112) is free of active chemicalcomponents such as oxygen. For example, the outer portion of thedielectric layer 114 may include silicon nitride and is free of oxygenor oxide. The dielectric layer 114 may include some oxide in the innerportion thereof in some embodiments. Alternatively, the dielectric layer114 may include one uniform layer of silicon nitride and is free ofoxide. The dielectric layer 114 may be deposited using CVD, PVD, ALD, orother suitable methods. In the present embodiment, the dielectric layer114 is deposited using ALD to ensure that it completely fills thetrenches 171.

At operation 216, the method 200 (FIG. 2C) forms another patterned mask172 over the device 100. The patterned mask 172 provides openings 173such as shown in FIGS. 12A and 12B, which are cross-sectional views ofthe device 100 along the B-B line and the C-C line of FIG. 1A,respectively. The openings 173 correspond to the dashed boxes 113 c ofFIG. 1A. Particularly, the openings 173 expose portions of the gatestructures 112 that are to be cut. The openings 173 may be formed by asingle patterning process or multiple patterning processes. The hardmask layer 172 may include titanium nitride, silicon nitride, amorphoussilicon, yttrium silicate (YSiO_(x)), or other suitable hard maskmaterial(s); and may be deposited using CVD, PVD, ALD, or other suitablemethods. In an example, the operation 216 may form a patternedphotoresist over the hard mask layer 172 by photoresist coating,exposing, post-exposure baking, and developing. Then, the operation 216etches the hard mask layer 172 using the patterned photoresist as anetch mask to form the opening 173. The etching process may include wetetching, dry etching, reactive ion etching, or other suitable etchingmethods. The patterned photoresist is removed thereafter, for example,by resist stripping.

At operation 218, the method 200 (FIG. 2C) etches the gate structures112 through the openings 173. Referring to FIG. 13 , the operation 218extends the opening 173 down and through the gate structures 112, andalso into the isolation structure 106 in an embodiment. The etchingprocess may use one or more etchants or a mixture of etchants that etchthe various layers in the gate structures 112. In an exemplaryembodiment, the conductive layer 110 includes TiSiN, TaN, TiN, W, or acombination thereof. To etch such a conductive layer and the high-kdielectric layer 108, the operation 218 may apply a dry etching processwith an etchant having the atoms of chlorine, fluorine, bromine, oxygen,hydrogen, carbon, or a combination thereof. For example, the etchant mayhave a gas mixture of Cl₂, O₂, a carbon-and-fluorine containing gas, abromine-and-fluorine containing gas, and a carbon-hydrogen-and-fluorinecontaining gas. In one example, the etchant includes a gas mixture ofCl₂, O₂, CF₄, BCl₃, and CHF₃. To ensure the isolation between theremaining portions of the gate structure 112, the operation 218 performssome over-etching to extend the openings 173 into the isolationstructure 106 in some embodiments. Such over-etching is carefullycontrolled to not expose the substrate 102.

At operation 220, the method 200 (FIG. 2C) fills the trenches 173 withone or more dielectric materials to form the dielectric features 114 c,and performs a chemical mechanical polishing (CMP) process to remove thepatterned hard mask 172 and to planarize the top surface of the device100.

The resultant structure is shown in FIGS. 14A and 14B which arecross-sectional views of the device 100 along the B-B line and the C-Cline of FIG. 1A, respectively. Particularly, the one or more dielectricmaterials in the trench 171 form the dielectric features 114 a and 114b, and the one or more dielectric materials in the trenches 173 form thedielectric features 114 c. Since the sidewalls of the gate structures112 contain metallic materials, at least the outer portion of thedielectric layer 114 (that is in direct contact with the sidewalls ofthe gate structures 112) is free of active chemical components such asoxygen. For example, the outer portion of the dielectric layer 114 mayinclude silicon nitride and is free of oxygen or oxide. The dielectriclayer 114 may include some oxide in the inner portion thereof in someembodiments. Alternatively, the dielectric layer 114 may include oneuniform layer of silicon nitride and is free of oxide. The dielectriclayer 114 may be deposited using CVD, PVD, ALD, or other suitablemethods. In the present embodiment, the dielectric layer 114 isdeposited using ALD to ensure that it completely fills the trenches 171and 173.

At operation 222, the method 200 (FIG. 2C) deposits a dielectric layer180 over the device 100, such as shown in FIG. 15 , which is across-sectional view of the device along the B-B line of FIG. 1A. In anembodiment, the dielectric layer 180 is another ILD layer and maycomprise TEOS oxide, un-doped silicate glass, or doped silicon oxidesuch as BPSG, FSG, PSG, BSG, and/or other suitable dielectric materials.The dielectric layer 180 may be formed by PECVD, FCVD, or other suitablemethods.

At operation 224, the method 200 (FIG. 2C) etches contact holes 182 intothe device 100, including contact holes 182 a exposing the S/D features162 a and contact holes 182 b exposing the S/D features 162 b, such asshown in FIG. 16 , which is a cross-sectional view of the device alongthe B-B line of FIG. 1A. In an embodiment, the operation 224 includescoating a photoresist layer over the device 100, exposing and developingthe photoresist layer to form openings, and etching the various layers180, 166, and 164 through the openings to form the contact holes 182.Particularly, the etching process is tuned to selectively etch thelayers 180, 166, and 164 but not the S/D features 162 and the dielectriclayer 114. The etching process is dry etching in an embodiment.

At operation 226, the method 200 (FIG. 2C) deposits one or moreconductive materials 184 into the contact holes 182, such as shown inFIG. 17 , which is a cross-sectional view of the device along the B-Bline of FIG. 1A. In an embodiment, the method 200 may form silicidefeatures over the exposed surfaces of the S/D features 162 beforedepositing the conductive materials 184. In an embodiment, theconductive materials 184 includes a barrier layer such as TaN or TiN anda metal fill layer such as Al, Cu, or W. The conductive materials 184may be deposited using CVD, PVD, plating, or other suitable methods.

At operation 228, the method 200 (FIG. 2C) performs a CMP process toremove excessive materials 184 and to expose the dielectric layer 114,such as shown in FIG. 18 , which is a cross-sectional view of the devicealong the B-B line of FIG. 1A. Referring to FIG. 18 , in the presentembodiment, the CMP process of the operation 228 separates theconductive materials 184 above the two S/D features 162 a to therebyform two S/D contacts that are isolated by the dielectric feature 114 a.Due to the large surface area of the S/D features 162 a, each of the twoS/D contacts has a sufficiently large interface with the underlying S/Dfeature 162 a for reducing S/D contact resistance.

At operation 230, the method 200 (FIG. 2C) performs further steps tocomplete the fabrication of the device 100. For example, the method 200may form metal interconnects electrically connecting the source, drain,gate terminals of various transistors to form a complete IC.

Although not intended to be limiting, one or more embodiments of thepresent disclosure provide many benefits to a semiconductor device andthe formation thereof. For example, embodiments of the presentdisclosure provide a two-step cut metal gate process where the firststep etches dielectric layers but not the metal gate and the second stepetches the metal gate. Embodiments of the present disclosure thenutilize the first etching step to separate previously merged S/Dfeatures that are designed for individual transistors. This allowssemiconductor fins for individual transistors to be arranged closer inembodiments of the present disclosure than in traditional devices andthe S/D features to be grown larger than traditional devices. This notonly increases device integration, but also reduces S/D contactresistance.

In one exemplary aspect, the present disclosure is directed to a method.The method includes providing a structure having a substrate and firstand second fins over the substrate and oriented lengthwise generallyalong a first direction; epitaxially growing semiconductor source/drain(S/D) features over the first and second fins, wherein a firstsemiconductor S/D feature over the first fin merges with a secondsemiconductor S/D feature over the second fin; and performing a firstetching process to an area between the first and second fins, whereinthe first etching process separates the first and second semiconductorS/D features.

In an embodiment, the method further includes, before the performing ofthe first etching process, forming gate structures over the substrateand the first and second fins, wherein the gate structures are orientedlengthwise generally along a second direction perpendicular to the firstdirection, wherein the first etching process is tuned to selectivelyetch the first and second semiconductor S/D features but not the gatestructures. In a further embodiment, wherein the forming of the gatestructures includes forming temporary gate structures over the substrateand the first and second fins; depositing a dielectric layer over thetemporary gate structures and the semiconductor S/D features; removingthe temporary gate structures, resulting in gate trenches in thedielectric layer; and depositing the gate structures in the gatetrenches. In a further embodiment, the first etching process is tuned toalso etch the dielectric layer. In a further embodiment, wherein theperforming of the first etching process results in a trench in thedielectric layer in the area between the first and second fins, themethod further includes depositing one or more dielectric materials inthe trench. In a further embodiment, the method further includes etchinga contact hole that exposes both the first and the second semiconductorS/D features; depositing a conductive material in the contact hole; andperforming a chemical mechanical planarization (CMP) process to separatethe conductive material into first and second portions, wherein thefirst and second portions are electrically connected to the first andsecond semiconductor S/D features respectively, and are isolated fromeach other by the one or more dielectric materials.

In another embodiment, the structure further includes a third fin overthe substrate and oriented lengthwise generally along the firstdirection; the gate structures are also formed over the third fin; andthe first etching process is also performed to an area between thesecond and the third fins. In a further embodiment, the method furtherincludes performing a second etching process to the area between thesecond and the third fins, wherein the second etching process is tunedto etch the gate structures.

In an embodiment of the method, the first and second semiconductor S/Dfeatures include p-type doped silicon germanium. In another embodiment,wherein the performing of the first etching process results in a trenchbetween the first and the second semiconductors S/D features, the methodfurther includes depositing one or more dielectric materials in thetrench.

In another exemplary aspect, the present disclosure is directed to amethod. The method includes providing a structure having: a substrate;first, second, and third fins over the substrate and oriented lengthwisegenerally along a first direction; gate structures over the first,second, and third fins and oriented lengthwise generally along a seconddirection perpendicular to the first direction; first and secondepitaxial semiconductor source/drain (S/D) features over the first andsecond fins respectively, wherein the first and second epitaxialsemiconductor S/D features merge along the second direction; and a firstdielectric layer over the substrate, the first, second, and third fins,and the first and second epitaxial semiconductor S/D features, andfilling space between the gate structures. The method further includesperforming a first etching process to a first area between the first andthe second fins and to a second area between the second and the thirdfins, wherein the first etching process is tuned to selectively etch thefirst and second epitaxial semiconductor S/D features and the firstdielectric layer but not the gate structures. The method furtherincludes performing a second etching process to the second area, whereinthe second etching process is tuned to selectively etch the gatestructures.

In an embodiment of the method, the first etching process separates thefirst and second epitaxial semiconductor S/D features. In anotherembodiment, wherein the first etching process results in a first trenchbetween the first and the second epitaxial semiconductor S/D features,the method further includes depositing one or more dielectric materialsin the first trench. In a further embodiment, the first and secondetching processes collectively form a second trench between the secondand third fins, and the one or more dielectric materials are alsodeposited in the second trench.

In an embodiment of the method, the first etching process includes dryetching with an etchant having hydrogen fluoride and ammonia. In anotherembodiment of the method, the second etching process uses achlorine-containing etchant.

In yet another exemplary aspect, the present disclosure is directed to asemiconductor structure. The semiconductor structure includes asubstrate; first and second fins over the substrate and orientedlengthwise generally along a first direction; first and second epitaxialsemiconductor source/drain (S/D) features over the first and second finsrespectively; and a first dielectric layer disposed between and inphysical contact with the first and the second epitaxial semiconductorS/D features, resulting in a first interface between the firstdielectric layer and the first epitaxial semiconductor S/D feature, anda second interface between the first dielectric layer and the secondepitaxial semiconductor S/D feature, wherein the first and secondinterfaces are tilted toward each other from top to bottom.

In an embodiment, the semiconductor structure further includes gatestructures over the first and second fins and oriented lengthwisegenerally along the second direction; and a second dielectric layer overthe substrate, the first and second fins, and the first and secondepitaxial semiconductor S/D features, and filling space between the gatestructures, wherein the first and the second dielectric layers includedifferent dielectric materials. In a further embodiment, thesemiconductor structure further includes a contact etch stop layerbetween the second dielectric layer and the first and second epitaxialsemiconductor S/D features.

In another embodiment, the semiconductor structure further includes afirst conductive feature over the first epitaxial semiconductor S/Dfeature; and a second conductive feature over the second epitaxialsemiconductor S/D feature, wherein the first dielectric layer isdisposed between the first and the second conductive features.

In an embodiment of the semiconductor structure, the first and secondinterfaces are two generally straight lines in a cross-sectional viewcut along a second direction perpendicular to the first direction. In afurther embodiment, each of the two generally straight lines forms anangle ranging from 0 to 5 degrees with normal of a top surface of thesubstrate.

In another embodiment of the semiconductor structure, each of the firstand second epitaxial semiconductor S/D features includes p-type dopedsilicon germanium.

In yet another embodiment, the semiconductor structure further includesa third fin over the substrate and oriented lengthwise generally alongthe first direction; and a third epitaxial semiconductor S/D featureover the third fin, wherein the first dielectric layer is also disposedbetween the second and the third epitaxial semiconductor S/D features.In a further embodiment, the first dielectric layer is not in directcontact with the third epitaxial semiconductor S/D feature. In anotherfurther embodiment, each of the first and second epitaxial semiconductorS/D features includes p-type doped silicon germanium; and the thirdepitaxial semiconductor S/D feature includes n-type doped silicon.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A semiconductor structure, comprising: anisolation structure; a first source/drain structure over the isolationstructure; a contact etch stop layer over the isolation structure andcontacting a first side face of the first source/drain structure; and afirst dielectric structure contacting a second side face of the firstsource/drain structure, wherein the first side face and the second sideface are on opposite sides of the first source/drain structure, and thefirst dielectric structure extends higher than the first source/drainstructure.
 2. The semiconductor structure of claim 1, furthercomprising: a first fin extending from a substrate and through theisolation structure, wherein the first source/drain structure is formedover the first fin, and wherein the first side face and the second sideface are on opposite sides of the first fin.
 3. The semiconductorstructure of claim 1, wherein the second side face forms an angleranging from 0 to 5 degrees with respect to a normal to a top surface ofa substrate over which the isolation structure is disposed.
 4. Thesemiconductor structure of claim 1, wherein the first side face is acrystalline facet of the first source/drain structure, and wherein thesecond side face is not a crystalline facet of the first source/drainstructure.
 5. The semiconductor structure of claim 1, wherein a bottomsurface of the first dielectric structure is below a top surface of theisolation structure.
 6. The semiconductor structure of claim 1, whereinthe first source/drain structure further includes a third side face thatis below the second side face, the second and the third side facesdisposed on a same side of the first source/drain structure, and thethird side face separated from the first dielectric structure.
 7. Thesemiconductor structure of claim 1, further comprising: a contactstructure electrically contacting a fourth side face of the firstsource/drain structure and contacting the first dielectric structure. 8.The semiconductor structure of claim 2, further comprising: a second finextending from the substrate and through the isolation structure; and asecond source/drain structure over the second fin, wherein the firstdielectric structure contacts a fifth side face of the secondsource/drain structure and extends higher than the second source/drainstructure.
 9. The semiconductor structure of claim 8, furthercomprising: a first contact structure electrically contacting a sixthside face of the first source/drain structure and contacting the firstdielectric structure; and a second contact structure electricallycontacting a seventh side face of the second source/drain structure andcontacting the first dielectric structure, wherein the first and thesecond contact structures are isolated from each other.
 10. Asemiconductor structure, comprising: an isolation structure; a pair offirst epitaxial semiconductor features over the isolation structure andhaving a first conductivity type; a pair of second epitaxialsemiconductor features over the isolation structure and having a secondconductivity type opposite to the first conductivity type; two contactfeatures on the pair of first epitaxial semiconductor featuresrespectively; and a first dielectric feature sandwiched between andseparating the pair of first epitaxial semiconductor features andsandwiched between and separating the two contact features, wherein thepair of second epitaxial semiconductor features merge with each other.11. The semiconductor structure of claim 10, wherein each of the pair offirst epitaxial semiconductor features includes silicon germanium, andeach of the pair of second epitaxial semiconductor features includessilicon.
 12. The semiconductor structure of claim 10, further comprisinga void that exposes a side surface of the first dielectric feature, atop surface of the isolation structure, and a side surface of one of thepair of first epitaxial semiconductor features.
 13. The semiconductorstructure of claim 12, further comprising another void that exposes thetop surface of the isolation structure and two side surfaces of the pairof second epitaxial semiconductor features.
 14. The semiconductorstructure of claim 10, wherein a bottom surface of the first dielectricfeature is disposed below a top surface of the isolation structure. 15.The semiconductor structure of claim 10, wherein the first dielectricfeature has a tapered profile with a top portion of the first dielectricfeature being wider than a lower portion of the first dielectricfeature.
 16. The semiconductor structure of claim 10, further comprisinga contact etch stop layer on a surface of the pair of first epitaxialsemiconductor features, wherein the contact etch stop layer is free froman area between the first dielectric feature and the pair of firstepitaxial semiconductor features.
 17. The semiconductor structure ofclaim 10, further comprising a second dielectric feature disposedbetween the pair of first epitaxial semiconductor features and the pairof second epitaxial semiconductor features, wherein a bottom surface ofthe second dielectric feature is disposed below a top surface of theisolation structure.
 18. A semiconductor structure, comprising: anisolation structure; first and second epitaxial semiconductor featuresover the isolation structure and having a first conductivity type; afirst dielectric feature between the first and the second epitaxialsemiconductor features, wherein a first portion of the first dielectricfeature contacts the first and the second epitaxial semiconductorfeatures, and a second portion of the first dielectric feature extendsbelow the first portion and does not contact the first and the secondepitaxial semiconductor features; a contact etch stop layer (CESL) on asurface of the first epitaxial semiconductor feature; and an interlayerdielectric layer over the contact etch stop layer.
 19. The semiconductorstructure of claim 18, wherein the first dielectric feature furtherincludes a third portion that extends above the first portion and doesnot contact the first and the second epitaxial semiconductor features.20. The semiconductor structure of claim 18, further comprising a seconddielectric feature that contacts a surface of the CESL, wherein thefirst and the second dielectric features include different materials.